With x-ray computed tomography for medical use, a three-dimensional image of the object is calculated by means of a specific method from standard x-ray images, which contain no depth information since they display a projection of a three-dimensional object on a two-dimensional plane. The x-ray tube and detector are hereby rotated about the object through at least 180° and projection images of the object are thereby recorded in small angular steps. A three-dimensional data set of the object can be achieved from these numerous projection images by means of special algorithms, e.g. the so-called filtered back projection. This imaging method is generally carried out using computed tomographs (CT) specially designed for this purpose.
Other x-ray devices allowing good access to the patient are often used for x-ray recordings during image-controlled diagnostic or surgical interventions on patients, in which standard x-ray images are continuously recorded during said intervention. The so-called C-arm systems, in which the x-ray tube and detector are arranged on opposing arms of a C-arm which can move freely about the patient, are favored here. Three-dimensional tomography images of the patient can optionally also be generated with such C-arm systems, since the C-arm can likewise move through approximately 180° about the patient. However the problem often arises here that the x-ray detector is relatively small, in particular smaller than with conventional computed tomographs, and thus parts of the object are truncated on some of the projection images, and therefore no longer lie in the field of view of each projection image. Artifacts result during the reconstruction of the projection images into a three-dimensional tomography image, since these parts of the object are in some projection images, but not others.
Methods to correct such truncated projection images (so-called “truncation correction”) are proposed, which hereby essentially reduce the image artifacts, by reducing the stage at which the image intensity suddenly drops to zero by virtue of the end of the field of view. The truncated projection image is supplemented, e.g. by means of linear extrapolation or another slow drop in the image intensity to zero, and these supplemented projection images are used for the reconstruction. Methods of this type are disclosed for instance in B. Ohnesorge et al., “Efficient correction for CT image artifacts caused by objects extending outside the scan field of view, “Med. Phys., vol. 27, no. 1, pp. 39-46, 2000 und R. R. Galigekere and D. W. Holsworth, “3D Reconstruction from Truncated Rotational Angiograms using Linear Prediction of view”, Proceedings of MICCAI 2003 pp. 126-133, 2003, as well as in the references of this article. These methods allow the image artifacts to be reduced, but an optimal correction is however not possible.